Underwater pipeline trenching apparatus

ABSTRACT

A pipeline trenching apparatus includes a carriage adapted to travel along the periphery of the pipeline. Rotary cutter drums advance with the carriage and are mounted upon frame elements pivotally attached to the carriage. Advancement of the apparatus is achieved by a reciprocating propulsion section connected to the carriage and provided with clamping elements alternately engaging and releasing the pipeline as the propulsion section is extended and retracted.

United States Patent 91 Lynch 541 UNDERWATER PIPELINE TRENCHINGAPPARATUS [76] Inventor: Robert P. Lynch, 5606 Farmwood Court,Alexandria, Va. 22310- 22 Filed: Aug. 19,1971

211 Appl.No.: 173,160

[52] US. Cl. ..61/72.4, 37/65, 254/105 [51] Int. Cl. ..E02f 5/02, E0211/00 [58] Field of Search ..61/72.4, 72.1, 72.3;

[56] References Cited UNITED STATES PATENTS Breston et a1. 61/72.4 DeVries ..61/724 1 May 15, 1973 3,429,132 2/1969 Martin ..61/72.43,616,651 11/1971 Chang et a1. ...61/72.3 2,879,649 3/1959 Elliott.....61/72.4 1,028,523 6/1912 Anderson ..6l/72.7 X

Primary Examiner-Jacob Shapiro Zitbrhey- Emory L. Groff et al.

"A EST'RKCT A pipeline trenching apparatus includes a carriage adaptedto travel along the periphery of the pipeline. Rotary cutter drumsadvance with the carriage and are mounted upon frame elements pivotallyattached to the carriage. Advancement of the apparatus is achieved by areciprocating propulsion section connected to the carriage and providedwith clamping elements alternately engaging and releasing the pipelineas the propulsion section is extended and retracted.

11 Claims, 12 Drawing Figures PATENTEWS'BB I 3.732.701

' SHEEIlUFS INVENTOR RO BERT F. LYNCH ATTORNEY 3 I 0 F, G 3s "1%? uni"$1 38 I 7/ 33' E INVENTOR M I ROBERT P. LYNCH ATTORNEY PATENTEDHAYISIHB01 ROBERT P LYNCH ATT EY PAIENIEMHS Y 3132.701

SHEET 5 OF 5 ZZ LLHHHI I FIG/0.

IINVENTOR R0 BERT P LY N CH,

ATTORNEY UNDERWATER PIPELINE TRENCIIING APPARATUS This invention relatesgenerally to pipeline machines, and more particularly, to an apparatusfor the placement of underwater pipelines in trenches.

Steel pipelines as utilized to transport oil or from offshore locationsto an onshore processing and distribution station are usually buried inthe ocean floor to prevent damage from bottom currents, anchors, fishinggear, and other potential hazards. The most common technique utilizedfor burying offshore pipelines involves the use of an apparatus such asa jet barge in combination with a submersible jet sled. The barge servesas a floating base and contains extensive apparatus including highpressure water pumping machines and suction dredge pumps, all of whichlead by means of often cumbersome lines to the jet sled, which comprisesa frame adapted to straddle a pipeline previously deposited upon theocean floor.

Many types of jet sleds are well known in the art, yet all of theseutilize a basic principle wherein high pressure water generated upon thebarge is transmitted to a plurality of nozzles on the sled so arrangedas to excavate a trench immediately below the pipeline. The disruptedsoil often is subsequently removed by means of a plurality of suctionpipes behind the jet nozzles which communicate with the barge-mounteddredge pumps.

Many disadvantages are inherent in the abovedescribed system, none ofthe least of which is the inefficiency of an arrangement wherein boththe water jet and suction forces are generated on the surface of thewater and must be transmitted through significant distances to the sleddevice located on the ocean floor. It will be appreciated that a strongsurface current and/or rough seas impose potential dangers to theconnecting lines between the barge and sled. Additionally, a problemoften arises in the means commonly used to advance the jet sled alongthe pipeline. This is usually effected by means of a cable systembetween the barge and sled and wherein the barge pulls itself bymeans ofan anchor winch, which operation is extremely hazardous and difficult tocontrol at any time when the surface of the water is not perfectly calm.The present invention seeks to provide an improved arrangement for theplacement'of underwater pipelines in trenches and includes a unitaryassembly comprising a pipe-riding carriage and a reciprocatingpropulsion section which is adapted to be initially placed over apreviously deposited pipeline on the ocean floor and which contains apair of mechanical cutter drums for digging a trench beneath thepipeline. The reciprocating propulsion section is automatically andsequentially clamped to the periphery of the pipeline while a fluidcylinder is actuated to advance the pipe-riding carriage as the cutterdrums are rotated to remove the soil beneath the pipeline.Self-contained suction dredge pumps associated with the cutter drumsremove the dislodged soil and direct it laterally of the movingapparatus. Additional means for facilitating the soil removal may beincluded in the form of water jet nozzles when the apparatus isemployedin an environment comprising clays of high cohesion such as the soilreferred to as gumbo and which will be found in the Gulf of Mexico,among other areas.

Accordingly, one of the objects of the present invention is to providean improved underwater pipeline trenching apparatus including apipe-riding carriage having a pair of cutter drums located at theforward end thereof and which are adapted to be lowered over apreviously deposited pipeline and subsequently pivoted inwardly withtheir lowermost portions positioned beneath the center of the pipeline.

Another object of the present invention is to provide a pipelinetrenching apparatus including cutter drums carried by a pipe-ridingcarriage which is sequentially advanced along the length of the pipelineas the cutter drums rotate, by means of a reciprocating propulsionsection connected to the carriage and including pipe clamping shoes.

A further object of the present invention is to provide a pipelineapparatus having a pipe-riding inner frame provided with a pair of outerframe sections pivotally attached thereto and each supporting a rotarycutter drum.

Still another object of the present invention is to provide a pipelinetrenching apparatus having a pipe-riding carriage with cutter meansthereon and including selfcontained advancing means comprising areciprocating pipe clamping section connected to the pipe-ridingcarriage.

With these and other objects in view which will more readily appear asthe nature of the invention is better understood, the invention consistsin the novel construction, combination, and arrangement of partshereinafter more fully described, illustrated, and claimed.

A preferred and practical embodiment of the present invention appears inthe accompanying drawings, in which:

FIG. 1 is a side elevation of the pipeline trenching apparatus of thepresent invention;

FIG. 2 is a top plan view of the apparatus shown in FIG. 1;

FIG. 3 is a rear end elevation taken along the line 3-3 of FIG. 1;

FIG. 4 is a transverse sectional view taken along the line 4-4 of FIG.1;

FIG. 5 is a front end elevation of the apparatus of FIG. 1 with thecutter drums removed for purposes of clarity;

FIG. 6 is a transverse sectional view taken along the line 6-6 of FIG.1;

FIG. 7 is an elevational view of one of the cutter drums with portionsbroken away to show the hollow interior of the cutter tube;

FIG. 8 is a view similar to FIG. 7 of thetube as it appears when rotatedninety degrees with respect toFIG.

FIG. 9 is a horizontal sectional view taken along the line 9-9 of FIG.7;

FIG. 10 is a horizontal sectional view taken along the line 10-10 ofFIG. 7;

FIG. 11 is a horizontal sdctional view taken along the line 11-11 ofFIG. 7; and

FIG. 12 is a horizontal sectional view taken along the line 12-12 ofFIG. 7.

Similar reference characters designate corresponding parts throughoutthe several figures of the drawings.

Referring now to the drawings, particularly FIGS. 1 and 2, the trenchingapparatus of the present invention will be seen to comprise a unitaryassembly including two joined principal components, namely, aforwardmost pipe-riding carriage generally designated 1, and rearmostreciprocating propulsion section 2. The apparatus .is illustrated inthese figures as it appears when disposed in its working positionwherein the weight of the machine as counterbalanced by means of asuitable ballast tank 3, is borne by means of a plurality of rollersengaging the periphery of the previously deposited pipeline P restingupon the ocean floor.

It will be appreciated that conventional means may be employed todeliver the apparatus from a surface craft to its use position. Suitablemeans may include a pair of guide wires leading from a surface barge tothe pipeline. After divers have connected the lower ends of the guidewires to the pipeline and the wires are tensioned, the trenchingapparatus may be lowered from the barge to the pipeline by fairleadingthe guide wires through appropriate guide shackles (not shown) on theapparatus.

The pipe-riding carriage 1 is constructed so as to permit readyplacement of the apparatus upon a previously deposited pipeline P. Thecarriage includes an inner frame 4 which is of generally rectangularconfiguration and which includes longitudinally extending side rails 5-5fixedly joined together by means of transverse end rails 66. Dependingfrom each side rail 5 are a plurality of inclined fixed support rollers7. By referring to these rollers as fixed", it is not meant to conveythe idea that the angular position of their axes is stationary since itis intended that suitable well-known means may be provided to permitselective adjustment of each roller axle according to the diameter ofthe particular pipeline being operated upon. As will be seen mostclearly in FIG. 6 of the drawings, the rollers 7 are preferably adjustedso that an extension of the plane of rotation of these rollers will passthrough the center axis of the particular pipeline being served.Notwithstanding the adjustable feature desirable with the fixed supportrollers 7, it will be understood that during the normal operation of theapparatus, the mounting axles of these rollers are maintained in a fixedposition with respect to the supporting inner frame 4.

Pivotally attached to each inner frame side rail 5, as at the pivotpoints 8, is an outer frame section 9 each comprising a generallyrectangular framework including a top rail 10, bottom rail 11, andvertical connecting rails 12-12. While the lateral extent of the innerframe 4 is substantially equal to the diameter of the pipeline P, itwill be noted that the overall vertical extent of the outer framesections 9-9 is significantly greater than the pipeline diameter.Depending downwardly and inwardly from the lower portion of each outerframe section 9 are a plurality of pivotal clamp rollers 13 providingguide means for the outer frame section. These are referred to aspivotal rollers in view of their radial or arcuate displacement duringattachment and removal of the apparatus from a pipeline, as illustratedin the alternate positions as shown in FIGS. 5 and 6 and which will be.described hereinafter. The axles of the pivotal clamp rollers 13, likethe axles of the fixed support rollers 7, may be connected to theirsupporting framework by any conventional means which will permit angularadjustment thereof so that when in the clamping position as shown inFIG. 6 an extension of the plane of rotation of these rollers willlikewise pass through the center axis of the pipeline P.

Means are provided on the pipe-riding carriage l to displace the outerframe sections 99 between the alternate positions of FIGS. 5 and 6.Preferably, this means comprises a pivot-clamping cylinder 14 such as apneumatic or hydraulic cylinder for the arcuate displacement of eachouter frame section with respect to the inner frame 4 and thus, thepipeline itself. As shown in FIGS. 2 and 6, the stationary end of eachcylinder 14 is pivotally attached to an anchor block 15 forming a partof the inner frame 4 while the free or operating end of the cylinder ispivotally attached as at 15a to the top rail 10 of one of the outerframe sections 9. With the foregoing arrangement in mind, it will beappreciated that, since the outer frame sections 9 are pivotallyattached to and supported by the inner frame 4 as pivot points 8 locatedbelow the top rails 10 of each outer frame section 9, upon actuation ofthe two pivotclamping cylinders 14, the pair of outer frame sectionswill be arcuately displaced either towards or away from one anotherabout their respective pivot points 88. During the initial placement ofthe apparatus upon the pipeline P, the two cylinders 14 are actuated inorder to collapse or retract their operating piston rods in order tomove the two outer frame section top rails 10-10 towards one another asin the condition shown in FIG. 5 whereupon the lower portions of theouter frame sections and their attached pivotal clamp rollers 13 will beparted from one another so that the apparatus may be lowered upon thepipeline until the fixed support rollers 7 are resting thereupon, afterwhich the fluid cylinders 14 are actuated in order to extend theiroperating rods to move the outer frame sections 9-9 to the workingposition of FIG. 6 as will be covered more in detail hereinafter.

In the present arrangement, the trench cutting action is achieved bymeans of a pair of cutter drums, generallydesignated C, one of which isprovided at the forwardmost extension of each other frame section 9. Thedetails of a preferred cutter drum are illustrated in FIGS. 7-12 of thedrawings, wherein it will be seen that each cutter drum C includes atube 16 comprising a hollow pipe member providing a continuous internalpassageway 17 extending from the bottom opening 18 of the tube throughthe top opening 19 thereof.

A cutter drum tube 16 is mounted adjacent the forward portion of eachouter frame section 9 by means of a pair of support arms 20-20 connectedto each vertical connecting rail 12 on the one hand and provided at theopposite end with a support collar 21, which collar surrounds the upperportion of the cutter tube 16. The actual cutting elements of eachcutter drum C comprise a plurality of angled cutter bars 22 attached tothe periphery of the cutter tube 16 in the cantilevered manner by meansof radially extending cutter bar brackets 23 as shown most clearly inFIGS. 7 and 8 of the drawings. The cross-sectional configuration of eachcutter bar itself will be most readily apparent from FIG. 11 wherein itwill be seen that the leading edge of each cutter bar 22 is providedwith a bevel 24 on its outermost surface. Each cutter bar is attached,by means of its pair of brackets 23, in an angular relationship withrespect to the cutter drum tube when viewed both in vertical as well ashorizontal plan. As shown in FIG. 11 each cutter bar is mounted with itsleading edge 25 disposed a greater radial distance from the center axisof the cutter drum than the trailing edge 26 thereof. Additionally, thelower portion 27 of each cutter bar is positioned at a closer radialdistance to the cutter tube than the upper portion 28 thereof. As shownin FIGS. 7 and 8, each cutter bar 22 is paired with an oppositelydisposed cutter bar mounted at the same angle and same distance from thecutter tube,

while the next lowest pair of cutter bars are disposed 90 with respectto the first-mentioned pair of cutter bars. This relationship continueswith the next succeeding lowermost pairs of cutter bars such that analternate arrangement will be seen to exist. Other than theabove-referenced offset or staggered relationship one other importantfeature should be recotnized with regard to the various cutter bars andthat is that each succeeding lowermost pairs of cutter bars 22 areattached to the cutter drum tube 16 by means of brackets 23 of adecreasing radius which radii are selected to provide a cutting pathwhen the drum is rotated which will describe a plurality of arcs all ofwhich will conform to the configuration of an inverted cone. Noting thatthe lower portion 27 of one bar is located in the same horizontal planeas the upper portion 28 of the next lowest cutter bar, it will thusfollow that, upon rotation of the cutter drum as it is lowered into anocean bed, aconically shaped cavity will be produced therein.

In the present apparatus a pair of the abovedescribed cutter drums areemployed, one of which is carried by each of the outer frame sections 9such that actuation of the pivot-clamping cylinders 14 produces aconcurrent tilting or lateral pivoting of the two cutter drums C-C. Thedisposition of the lowermost pair of opposed cutter bars 2222 will be ofparticular concern when considering the operation of the apparatus nowbeing described. As will be seen the lower portion 27 of these lowermostcutterbars extends well below the bottom opening 18 of the cutter drumtube 16 so that at all times during operation of the apparatus thesebottom cutter bars will insure that the bottom opening of the tube 18will be spaced from the side and bottom walls of the trench T.

The mechanism associated with the operation of the described cutterdrums will now be covered. As previously set forth, a complete cutterdrum assembly is mounted forward of the vertical connecting rail 12 ofeach one of the outerframe sections 9 and supported in this position bymeans of the bearings or support collars 21-21. These collars 21 providethe entire sup porting means for the cutter drum'and its variousrotating components. The uppermost extremity of the cutter drum tube 16is journaled within the pump collar 29 (FIG. 1) which is fixedlyattached to the forwardmost limit of the top rail of its respectiveouter frame section 9. Carried by each pump collar 29 is a suctiondredge pump 30 which may be of any suitable wellknown construction butwhich is preferably driven by an appropriate hydraulic motor. Thenegative suction of the pump 30 will be understood to communicate withthe top opening 19 of the cutter drum tube 16 while the outlet ordischarge of the pump is directed through the discharge tube 31 havingits exit portion 32 extending in a direction away from the center lineof the pipeline P and preferably rearwardly of the cutter drum C. Withthe foregoing structure in mind it will be appreciated that, uponactuation of the suction dredge pumps 30, a negative pressure will becreated in the area of the bottom opening 18 of each cutter drum tube 16such that a suspension of any dislodged soil and rocks in the area ofthe lower portion of each cutter drum will be drawn upwardly through theinternal passageway 17 by the pump and thence directed outwardly throughthe exit portion 32 thereof.

Any suitable means may be provided to impart a simple rotary motion tothe cutter drum C-C. Preferably,

another hydraulic motor (not shown) is provided on each outer framesection 9 including a gear-drive which may mesh either directly or bymeans of a chain with the cutter drum drive gear 33 fixedly attached toeach cutter drum tube 16 in the area adjacent the uppermost supportcollar 21.

The operation of the structure described up to this point may now berelated. As the trenching apparatus is lowered from the surface barge tothe previously deposited pipeline P it is guided so that the openedouter frame sections 9 and their respective cutter drums will straddlethe pipeline. It will be apparent that initially the apparatus mustassume a three-point position of rest, that is, the rearmost portion ofthe apparatus will be disposed upon the pipeline while the lowermostportion of the two spread-apart cutter drums engage the ocean floor.Before the apparatus can assume the position as shown in FIGS. 1 and 2,the operator initiates rotation of the two cutter drums and theoperation of their respective dredge pumps 30, whereupon the apparatusin actuality digs its own way into the normal horizontal position untilthe fixed support rollers 7 engage the upper portion of the pipelineperiphery. During this digging-in procedure and when the uppermost pairsof cutter bars 22 have been lowered to a point below the midline of thepipeline, the operator actuates the fluid cylinders 14-l4 toprogressively close the outer frame sections 99 whereby, when theapparatus is fully supported on the pipeline by means of the rollers 7,the outer frame sections 9-9 will be clamped to the pipeline by means ofthe rollers 13 and the components will appear as in FIGS. 3 and 6.

A unique arrangement is included to provide the selfcontained advancingmechanism. Extending from the rear of the inner frame 4 are a pair ofparallel longitudinal guide members 34--34 having their forward portionsrigidly attached to the rear end rail 6 and their distal portionsconnected by means of the cross bar 35. Associated with the pair oflongitudinal guide members 34-34 is the reciprocating propulsion section2 having a rigid shoe frame 36 attached to the guide members 34 by meansof a plurality of slide blocks 37 journalled about each of the guidemembers 34. The shoe frame 36 will thus be seen to be capable only oflongitudinal reciprocating motion within the limits of the extent of theguide members 34 and is provided to serve as a movable support for apair of movable pipe clamping shoes 3838. Each shoe 38 comprises anarcuate plate having a radius of curvature substantially similar to theradius of pipeline P being operated upon and is adapted to engage asignificant portion of the periphery of the pipeline on opposite sidesof its vertical center line. The supporting and operating structureassociated with the pipe clamping shoes is shown most clearly in FIGS. 2and 4 wherein it will be seen that an upper portion of each shoe isrigidly connected to a shoe link 39, the upper extremities of which arepivotally connected as at 40 to the top center portion of the shoe frame36. Pivotally anchored at 41 to the side bar 42 of the shoe frame 36 isa shoe actuating cylinder 43 which has the distal portion of its movablepiston rod pivotally connected to the outer periphery of each shoe 38 asat 44. It will thus be apparent that, upon actuation of the two shoeactuating cylinders 43-43, the respective pipe clamping shoes 3838 maybe selectively manipulated between the clamping position of FIG. 3 andthe open position of FIG. 4 of the drawings. With the cylinders 43extended and with a positive pressure forcing the two clamping shoestightly against the periphery of the pipeline, as in FIG. 3, it will beappreciated that the entire shoe frame assembly 36 is rigidly anchoredto the pipeline P. With the apparatus in this condition, as shown inFIGS. 1 and 2, and with the cutter drums C rotating, the entirepipe-riding carriage 1 may be advanced longitudinally of the pipeline bythe actuation of a propulsion fluid cylinder 45, one end of which isrigidly secured to the inner frame 4 of the carriage 1, while its otherend is attached to the propulsion section 2. Thus, self-contained meansare provided to advance the cutter bar-equipped pipe-riding carriage 1within the limits of the longitudinal extent of the two guide members34-34. With the propulsion cylinder 45 fully extended and the rearportion of the shoe frame 36 juxtaposed the cross bar 35, the twoshoe-actuating cylinders 4343 are collapsed a sufficient amount torelease the clamping pressure of the shoes upon the pipeline, whereuponthe propulsion cylinder 45 may be retracted to move the shoe frame 36 toits forwardmost limits as defined by the collapsed position of thecylinder 45, after which the shoe actuating cylinders 43 are againactuated to clamp the shoes 38 to the pipeline and the previouslydescribed procedure is repeated.

When the cutter drums are revolving in a contrarotary manner it will beunderstood that the longitudinal axes of the two drums will be slightlyinclined towards one another, from the carriage 1 to the bottom of thetrench T, as indicated by the cutter drum axis lines 4646 illustrated inFIG. 3. Wlth this arrangement the cutter bars 22 which are closest tothe vertical central axis 47 of the pipeline on one cutter drum will beparallel to and closely spaced from the cutter bars of the other cutterdrum, such that a minimum space is presented between the two inclinedcutter drums. With the disclosed structure of cutter bars it will beappreciated that only two cutter bars on each cutter drum will beshaving the trench bank at any one time, thereby insuring a maximumcutting torque being concentrated and distributed to the single pair ofcutter bars engaging the trench wall. The described spaced-apart andstaggered arrangement of the cutter bars also encourages the unimpededdescent of the dislodged soil and rocks to the bottom of the trench Tduring operation of the apparatus. As the machine progresses along thepipeline with the cutter drums revolving, the dislodged soil and rocksare continuously falling by gravity to the bottom of the trench and intothe suction zone 48 located below the bottom opening 18 of the cutterdrum torque tube 16. Maintenance of the suction zone 48 is insured bymeans of the arrangement of the lowermost cutter bars 22 on each cutterdrum, since the lower portion 27 of these last-mentioned cutter bars isspaced well below the bottom opening 18 of the tube 16 so that thedislodged material may be readily drawn upwardly through the opening 18as a result of the suction action generated by the dredge pumps 30.

When the apparatus is being operated in an environment comprising claysof high cohesion, such as gumbo, there is a possibility that, as thecutter bars 22 shave away portions of the trench wall during advancementof the machine, large chunks of the clay may become lodged within theareas between the cutter bars 22, their supporting brackets 23, and theperiphery of the torque tube 16. For this reason it is proposed toprovide one or more water jet nozzles 49 with their exit portionsdirected to the cutter drums. As shown in FIG. 1, these nozzles 49 arepreferably attached to the lower portion of each outer frame section 9by means of a suitable bracket 50, and it will be understood that waterunder sufficient pressure is supplied to these nozzles from the surfacebarge communicating with the apparatus. Thus, as the drums revolve, itwill be seen that high speed water jets will impinge upon the cutterdrums to knock off any lodged material therein, causing this material tobreak up and subsequently fall by gravity into the influence of thesuction zone 48. By mounting the water jet nozzles 49 on the lowerportion of the outer frame sections 9 it will follow that the force ofthe jetted water will at all times impinge upon the cutter drumsregardless of the inclination of the cutter drums, since these drums arelikewise attached to the outer frame sections.

All of the hydraulic and other forces required to operate the variousmechanisms included in the present apparatus may be supplied theretofrom a suitable surface barge by means of a surface line or umbilicalcord 51, as shown in FIG. 3. Additionally, it is desirable to includesuitable manual switches or valves (not shown) on the apparatus itselfto enable a diver to actuate the various components of the device. Whena pipeline P has been buried and it is desired to remove the apparatustherefrom, the procedure involved is exactly the reverse of thatutilized in attaching the machine. With the cutter drums revolving, theforward portion of the machine is lifted by suitable cable means whilethe pivot/- clamping cylinders 14 are actuated to spread apart the outerframe sections and their attached cutter drums. When the pipe-ridingcarriage 1 is thus clear of the pipeline, the outer frame sections 9 maybe reclosed as the apparatus is being raised to the surface of thewater.

I claim:

1. An underwater pipeline trenching apparatus comprising, a carriageunit adapted for support upon a pipeline disposed on the floor of a bodyof water, said carriage including an inner frame section and a pair ofouter frame sections, a plurality of support rollers on said inner framesection engageable with the periphery of the pipeline when said carriageunit is in its normal working position, said outer frame sectionspivotally attached to said inner frame section for movement between anopen position and a clamping position with respect to said pipeline,guide means on said outer frame sections engageable with said pipelinewhen said outer frame sections are in the clamping position, a rotarycutter drum attached to each outer frame section, a propulsion sectiondisposed adjacent one end of said carriage unit, reciprocating drivemeans connected to said propulsion section for alternately moving saidpropulsion section towards and away from said carriage unit, guidemembers attached to said carriage unit and longitudinally extending fromone end thereof, slide means on said propulsion section engaging saidguide members, and clamping means on said propulsion section selectivelyengageable with the pipeline to lock said propulsion section theretowhereby, said drive means may be actuated to propel said carriage unitalong said pipeline.

2. An underwater apparatus according to claim 1 wherein, said cutterdrums are attached to the forward end of said outer frame sections andsaid propulsion section is joined to the rear of said carriage unit.

3. An underwater apparatus according to claim 1 including, afluid-actuated cylinder connected between each said outer frame sectionand said inner frame section for selectively displacing said outer framesections between the open and clamping positions.

4. An underwater apparatus according to claim 1 wherein, said innerframe section is normally disposed above the top of said pipeline andsaid support rollers engage the upper portion of the pipeline periphery,and said outer frame section guide means is disposed on the lowerportion of said outer frame sections and engages the-lower portion ofthe pipeline periphery when in the clamping position.

5. An underwater apparatus according to claim 1 wherein, each saidcutter drum includes a central tube having an internal passagewayextending from its bottom opening to its top opening, and a plurality ofcutter bars adjacent the exterior of said tube and connected thereto bymeans of outwardly extending brackets.

6. An underwater apparatus according to claim 5 including, pairs ofdiametrically opposed cutter bars projecting from said tube, andsuccessive pairs of said bars are angularly offset with respect to oneanother.

7. An underwater apparatus according to claim 6 including, pairs of saidcutter bars successively angularly offset from one another from theuppermost to the lowermost of said pairs.

8. An underwater apparatus according to claim 6 including, a pair ofsaid cutter bars extending substantially below said bottom opening.

9. An underwater apparatus according to claim 1 including, water jetnozzles supported upon said carriage and directed towards each saidcutter drum.

10. An underwater pipeline trenching apparatus comprising, a carriageunit adapted for support upon a pipeline disposed on the floor of a bodyof water, said carriage including an inner frame section and a pair ofouter frame sections, a plurality of support rollers on said inner framesection engageable with the periphery of the pipeline when said carriageunit is in its normal working position, said outer frame sectionspivotally attached to said inner frame section for movement between anopen position and clamping position with respect to said pipeline, guidemeans on said outer frame sections engageable with said pipeline whensaid outer frame sections are in the clamping position, a rotary cutterdrum attached to each outer frame section, a propulsion section disposedadjacent one end of said carriage unit, reciprocating drive meansconnected to said propulsion section for alternately moving saidpropulsion section towards and away from said carriage unit, andclamping means on said propulsion section selectively engageable withthe pipeline to lock said propulsion section thereto, said propulsionsection clamping means including a pair of arcuate shoes pivotallymounted on said propulsion section, a fluidactuated cylinder connectedto each said shoe for displacing same between its locking and releasedpositions whereby, said drive means may be actuated to propel saidcarriage unit along said pipeline.

11. An underwater pipeline trenching apparatus comprising, a carriageunit adapted for support upon a pipeline disposed on the floor of a bodyof water, said carriage including an inner frame section and a pair ofouter frame sections, a plurality of support rollers on said inner framesection engageable with the periphery of the pipeline when said carriageunit is in its normal working position, said outer frame sectionspivotally attached to said inner frame section for movement between anopen position and a clamping position with respect to said pipeline,guide means on said outer frame sections engageable with said pipelinewhen said outer frame sections are in the clamping position, a rotarycutter drum attached to each outer frame section, each said cutter drumincludes a central tube having an internal passageway extending from itsbottom opening to its top opening, and a plurality of cutter barsadjacent the exterior of said tube and connected thereto by means ofoutwardly extending brackets, a suction dredge pump mounted adjacent theupper portion of said tube and communicating with said top opening,propulsion section disposed adjacent one end of said carriage unit,reciprocating drivemeans connected to said propulsion section foralternately moving said propulsion section towards and away from saidcarriage unit, and clamping means on said propulsion section selectivelyengageable with the pipeline to lock said propulsion section theretowhereby, said drive means may-be actuated to propel'said carriage unitalong said pipeline.

1. An underwater pipeline trenching apparatus comprising, a carriageunit adapted for support upon a pipeline disposed on the floor of a bodyof water, said carriage including an inner frame section and a pair ofouter frame sections, a plurality of support rollers on said inner framesection engageable with the periphery of the pipeline when said carriageunit is in its normal working position, said outer frame sectionspivotally attached to said inner Frame section for movement between anopen position and a clamping position with respect to said pipeline,guide means on said outer frame sections engageable with said pipelinewhen said outer frame sections are in the clamping position, a rotarycutter drum attached to each outer frame section, a propulsion sectiondisposed adjacent one end of said carriage unit, reciprocating drivemeans connected to said propulsion section for alternately moving saidpropulsion section towards and away from said carriage unit, guidemembers attached to said carriage unit and longitudinally extending fromone end thereof, slide means on said propulsion section engaging saidguide members, and clamping means on said propulsion section selectivelyengageable with the pipeline to lock said propulsion section theretowhereby, said drive means may be actuated to propel said carriage unitalong said pipeline.
 2. An underwater apparatus according to claim 1wherein, said cutter drums are attached to the forward end of said outerframe sections and said propulsion section is joined to the rear of saidcarriage unit.
 3. An underwater apparatus according to claim 1including, a fluid-actuated cylinder connected between each said outerframe section and said inner frame section for selectively displacingsaid outer frame sections between the open and clamping positions.
 4. Anunderwater apparatus according to claim 1 wherein, said inner framesection is normally disposed above the top of said pipeline and saidsupport rollers engage the upper portion of the pipeline periphery, andsaid outer frame section guide means is disposed on the lower portion ofsaid outer frame sections and engages the lower portion of the pipelineperiphery when in the clamping position.
 5. An underwater apparatusaccording to claim 1 wherein, each said cutter drum includes a centraltube having an internal passageway extending from its bottom opening toits top opening, and a plurality of cutter bars adjacent the exterior ofsaid tube and connected thereto by means of outwardly extendingbrackets.
 6. An underwater apparatus according to claim 5 including,pairs of diametrically opposed cutter bars projecting from said tube,and successive pairs of said bars are angularly offset with respect toone another.
 7. An underwater apparatus according to claim 6 including,pairs of said cutter bars successively angularly offset from one anotherfrom the uppermost to the lowermost of said pairs.
 8. An underwaterapparatus according to claim 6 including, a pair of said cutter barsextending substantially below said bottom opening.
 9. An underwaterapparatus according to claim 1 including, water jet nozzles supportedupon said carriage and directed towards each said cutter drum.
 10. Anunderwater pipeline trenching apparatus comprising, a carriage unitadapted for support upon a pipeline disposed on the floor of a body ofwater, said carriage including an inner frame section and a pair ofouter frame sections, a plurality of support rollers on said inner framesection engageable with the periphery of the pipeline when said carriageunit is in its normal working position, said outer frame sectionspivotally attached to said inner frame section for movement between anopen position and clamping position with respect to said pipeline, guidemeans on said outer frame sections engageable with said pipeline whensaid outer frame sections are in the clamping position, a rotary cutterdrum attached to each outer frame section, a propulsion section disposedadjacent one end of said carriage unit, reciprocating drive meansconnected to said propulsion section for alternately moving saidpropulsion section towards and away from said carriage unit, andclamping means on said propulsion section selectively engageable withthe pipeline to lock said propulsion section thereto, said propulsionsection clamping means including a pair of arcuate shoes pivotallymounted on said propulsion section, a fluid-actuated cylinder conNectedto each said shoe for displacing same between its locking and releasedpositions whereby, said drive means may be actuated to propel saidcarriage unit along said pipeline.
 11. An underwater pipeline trenchingapparatus comprising, a carriage unit adapted for support upon apipeline disposed on the floor of a body of water, said carriageincluding an inner frame section and a pair of outer frame sections, aplurality of support rollers on said inner frame section engageable withthe periphery of the pipeline when said carriage unit is in its normalworking position, said outer frame sections pivotally attached to saidinner frame section for movement between an open position and a clampingposition with respect to said pipeline, guide means on said outer framesections engageable with said pipeline when said outer frame sectionsare in the clamping position, a rotary cutter drum attached to eachouter frame section, each said cutter drum includes a central tubehaving an internal passageway extending from its bottom opening to itstop opening, and a plurality of cutter bars adjacent the exterior ofsaid tube and connected thereto by means of outwardly extendingbrackets, a suction dredge pump mounted adjacent the upper portion ofsaid tube and communicating with said top opening, propulsion sectiondisposed adjacent one end of said carriage unit, reciprocating drivemeans connected to said propulsion section for alternately moving saidpropulsion section towards and away from said carriage unit, andclamping means on said propulsion section selectively engageable withthe pipeline to lock said propulsion section thereto whereby, said drivemeans may be actuated to propel said carriage unit along said pipeline.